Maliks Muwatta Book 9, Hadith Number 27.

Section : Voluntary Prayers while Travelling, by Day and at Night, and Praying on a Riding Beast.

Yahya related to me from Malik from Amr ibn Yahya al-Mazini from Abu’l-Hubab Said ibn Yasar that Abdullah ibn Umar said, “I saw the Messenger of Allah, may Allah bless him and grant him peace, praying on a donkey while heading towards Khaybar.”